Low Pass Filter Capacitors

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simon
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#16

Post by simon »

I have some rather tiny PP caps and some mica which are altogether easier to handle as they have nice long legs :-). So I soldered in the micas and the buffer hums like a good un with them in place! Really quite unpleasant. Don't know whether the micas are reacting badly (why would that be?) or there's something more fundamental. I've unsoldered one end of the caps and it's fine again. Tried clipleading each cap in turn but each one causes hum. Oh well, can't win them all.

The circuit (and accompanying spiel) is the second one down here if anyone has any thoughts:
http://tubecad.com/2012/05/blog0229.htm
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Mike H
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#17

Post by Mike H »

If it didn't hum before, for any other reason, then adding the caps may have inadvertently made a RF oscillator. Only thing I can readily think of that makes sense.
 
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simon
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Posts: 5600
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

#18

Post by simon »

It's an odd one Mike. I've followed JRB's recipe with 10K resistors and 100pF caps, but it really isn't happy so I'll leave them out unless I get some inspiration from somewhere.

I could try emailing him, but frankly there's little chance he'd answer as he reckons to drown in email.
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